1. Field of the Invention
The present invention relates to a voltage measuring device, in particular, for measuring a voltage of a high voltage battery mounted on a vehicle composed of a plurality of unit cells connected to each other in series.
2. Description of the Related Art
These days, a hybrid electric vehicle (hereunder referred to as “HEV”) using an electric motor together with an engine becomes widely used. This HEV includes a low voltage battery such as 12 V for starting the engine, and a high voltage battery for driving the electric motor. The high voltage battery obtains a high voltage by a plurality of unit cells connected in series. The unit cell is composed of one or more secondary batteries such as a nickel hydride battery or a lithium battery.
In the high voltage battery used for the HEV, it is necessary to measure a voltage of each unit cell for checking a charging condition of each unit cell. These days, in accordance with the performance increase of the HEV, the number of the cells of the high voltage battery is increased, and checking switches for measuring the voltage are increased. In view of this increase, by an integration of the voltage measuring device including the checking switches into an IC (integrated circuit), the number of parts of the voltage measuring device is decreased.
In the case of the integration of the voltage measuring device, Patent Document 1 discloses a method for measuring a voltage of each unit cell in each block consisting of a plurality of unit cells by mounting an IC in each block, and the IC including a switching circuit.
[Patent document 1] Japanese Published Patent Application No. 2005-318751
However, in the above-described case that the IC is mounted on each block, the number of unit cells in the high voltage battery should be a multiple number of checking points of the unit cells per one IC. However, owing to a battery capacity or a performance requirement of a vehicle, the number of the unit cells in the high voltage battery may disagree with the multiple number of the checking points of the unit cells per one IC. For example, if the number of the unit cells of the high voltage battery is fifty, and the checking points of the unit cells per one IC is four, two unit cells will remain. In this case, some measures are needed, for example, increasing the number of the unit cells of the high voltage battery to be a multiple of 4, or preparing another IC having two checking points, or a discrete voltage measuring device for two unit cells.
If the number of unit cells is increased, the voltage of the high voltage battery may not be the most suitable for a vehicle. Further, using another IC or using a discrete voltage measuring device causes an increase of the number of the parts or an increase of a mounting area. Thus, in both cases, a cost of the voltage measuring device increases.
Accordingly, an object of the present invention is to provide a voltage measuring device and a voltage measuring system to allow a suitable design corresponding to the most suitable number of the unit cells of the high voltage battery mounted on a vehicle with no increase in cost.